Guide to exotic and how to differentiate from thistles of Montana native thistles by Hilary Parkinson former Plant Identification Diagnostician, and Jane Mangold EB0221 Extension Invasive Plant Specialist COVER PHOTOS: Left: Bull thistle, page 5 Five exotic and ten native thistles grow in Montana. This publication is photo by Starr Environmental, bugwood.org designed to determine whether an unknown thistle is exotic or native; if Right: Flodman’s thistle, page 12 exotic, the publication will help you determine it to species. Based on the photo by M. Lavin, Montana State University technical nature and difficulty of identifying native thistles, they are not identified to species in the dichotomous key on page 3. This publication also includes instructions on how to use a dichotomous key, descriptive text, and pictures to illustrate the five exotic and ten native thistles that grow in Montana. table of contents Why it is important to identify exotic thistles and differentiate them from natives . 1 What to look for . 1 Tutorial on using a dichotomous key . 2 Thistle dichotomous key. 3 EXOTIC THISTLES introduction . 4 Bull . 5 Canada. 6 Musk . 7 Plumeless . 8 Scotch . 9 NATIVE THISTLES introduction . 10 Clustered. 11 Eaton’s . 11 Flodman’s . 12 Graygreen . 12 Longstyle . 13 Meadow or Elk . 13 Prairie . 14 Wavyleaf . 14 White or Elk . 15 The U.S. Department of Agriculture (USDA), Montana State University and Montana State University Extension prohibit discrimination in all of their programs and activities on the basis of race, color, national origin, gender, religion, age, disability, Wyoming . 15 political beliefs, sexual orientation, and marital and family status. Issued in furtherance of cooperative extension work in agriculture and home economics, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Sources. inside back cover Jeff Bader, Director of Extension, Montana State University, Bozeman, MT 59717. 1 2 Why is it important to identify exotic thistles and differentiate them from natives? 3) Assess whether the plant is rhizomatous or taprooted. Rhizomatous plants Exotic thistles can spread quickly (especially with disturbance), have poor send out shoots below the ground that develop into new plants at some forage value, and their sharp spines can limit recreational activities and distance from the mother plant. Rhizomatous plants will typically be more injure livestock. From a management perspective, differentiating among dense (spaced closer together) and when mature will lack a prominent exotics is important because perennial rhizomatous thistles will require basal rosette (clusters of leaves growing at the plant’s base). There are two different control measures than taprooted annuals or biennials. Compared thistles in Montana that are rhizomatous or may appear rhizomatous. to exotics, native thistles do not spread quickly with disturbance, are rarely Canada thistle is densely rhizomatous. Flodman’s thistle, can spread by reported as invasive, and are important for wildlife. For example, birds feed horizontal roots, so may at times grow more densely, thereby appearing on thistle seed, and some may time their nesting around thistle flowering, rhizomatous. using plumes on seeds to line nests. Bees, wasps, flies and beetles feed on 4) Assess whether flowers are densely clustered on short stalks or more solitary thistle pollen and become food sources for wildlife. For some large ungulates on long stalks. This characteristic is somewhat variable among the exotic like elk, native thistles are a source of forage. This guide serves to promote thistles and should be used in conjunction with other characteristics. See the importance of maintaining native thistles in the landscape and should Canada thistle, page 6, for an example of densely clustered flowers on be used to verify whether a thistle is native or exotic before implementing short stalks and musk thistle, page 7, for an example of solitary flowers on control strategies. long stalks. WHAT TO LOOK FOR TUTORIAL ON USING A DICHOTOMOUS KEY The features described below are essential for Dichotomous keys consist of a series of two mutually exclusive statements, determining whether the thistle is exotic or native. known as couplets. Starting with the first couplet, choose the statement that Become familiar with and carefully investigate the best fits the unknown plant; the corresponding number will lead to another features below before starting the dichotomous couplet, and ultimately to the plant’s name. Below is an example. key on page 3. 1a. Plant has spiny winged stems entire length . 2 1) Observe whether there are spiny wings along the entire stem length. If prominent, spiny wings 1b. Plant does not have spiny winged stems entire length . 5 (Figure 1) line the entire length of the stem The two choices, 1a and 1b, form a couplet. Imagine your unknown plant then it is an exotic thistle. If a few spines or does have spiny winged stems the entire length, meaning you should proceed spiny wings occur intermittently along the stem to couplet 2. then it could be native or exotic. FIGURE 1. Spines extend along entire length of stem. 2a. Flowers less than 1 inch in diameter and typically densely 2) Observe the bracts on the flowering head. Note clustered on short stalks . Plumeless thistle the height of the bracts collectively. Note the width of the flowering head. Thistles have bracts, 2b. Flowers more than 1 inch in diameter and tending to be solitary or tiny, leaf-like structures that form a cup on longer stalks . 4 around the base of flowers. The white arrow If the plant has small flowers densely clustered on short stems, then choose in Figure 2 points to a single bract. The shape 2a which leads to ‘plumeless thistle’. If flowers are larger and mostly solitary of an individual thistle bract, e.g. whether on longer stems, then choose 2b and proceed to couplet 4. Read each needle-like (e.g. bull thistle), or more broad couplet carefully before proceeding to the next step. It may take many steps and triangular (e.g. musk thistle) is important and some backtracking to arrive at the correct plant. for identification. Measure whether bracts collectively are greater than or less than 0.8 inches high, referring to the white bracket in Figure 2 for the length to measure. Measure the width of the flowering head, referring to FIGURE 2. How to measure bracts the black bract in Figure 2 for the length to collectively, an individual thistle measure. bract and head width. 3 4 SIMPLIFIED DICHOTOMOUS KEY FOR THISTLES OF MONTANA: IS IT EXOTIC OR NATIVE? IF EXOTIC, WHICH SPECIES? exotic thistles 1a. Spiny wings (Figure 1) consistently line entire stem length . 2 1b. Stems without spiny wings lining entire length; may be intermittent spines and/or spiny The following pages give detailed information and photos on the five wings, but not a consistent line from below flower heads to plant base . 5 exotic thistles that occur in Montana including life cycle, habitat, and key diagnostic features to identify them. 2a. Flower heads not more than 1 inch in diameter, bracts collectively less than 0.8 inches high Exotic thistles can form dense infestations, often in disturbed areas. and flowers tending to be on shorter stalks and in denser clusters, plant not silvery gray . Plumeless thistle, Carduus acanthoides, p. 8, Figure 6A 2b. Flower heads more than 1 inch in diameter, bracts collectively more than 0.8 inches high and flowers typically solitary on longer stalks; foliage green or silvery gray . 3 3a. Foliage silver gray in appearance, plants may grow up to 6 feet tall . Scotch thistle, Onopordum acanthium, p. 9, Figures 7B,C 3b. Foliage green, not silver gray, plants rarely over 5 feet tall . 4 4a. Floral bracts broad and triangular, and margins entire, bract midrib prominent, heads often nodding . .Musk thistle, Carduus nutans, p. 7, Figures 5A,B musk thistle S. Dewey, bugwood.org 4b. Floral bracts narrow, needle-like . Bull thistle, Cirsium vulgare, p. 5, Figure 3A 5a. Plants strongly rhizomatous, bracts collectively less than 0.8 inches high; (see the native Wyoming thistle with bracts collectively less than 0.8 inches high, but lacking rhizomes, and leaves white beneath) . Canada thistle, Cirsium arvense, p. 6, Figures 4A,B 5b. Plants not rhizomatous (or weakly showing signs of being rhizomatous), and/or bracts collectively more than 0.8 inches high . 6 6a. Plants with broad triangular bracts, often bent outwards from base, bract margins entire; spiny wings on stem may be absent below flower head, but begin some distance below and bull thistle J. DiTomaso, bugwood.org continue to plant base . Musk thistle, Carduus nutans, p. 7, Figures 5A,B 6b. Plants without broad triangular bracts . 7 7a. Upper leaf surface with many short, sharp spines covering all parts of this plant making it difficult to find any place to touch it without being injured, and outer bracts tend to point outward rather than upward . Bull thistle, Cirsium vulgare, p. 5, Figure 3C 7b. Upper leaf surface lacking spines and may be hairless or hairy; bracts tend to point upward, adhering to the flower head for most of their length (excluding some with a spine at the tip that may point outwards) . a native thistle* Scotch thistle J. Randall, bugwood.org * If you’d like to identify native thistle to species, refer to Manual of Montana Vascular Plants (Lesica 2012) or Vascular Plants of Montana (Dorn 1984), or submit samples to the MSU Schutter Diagnostic Lab (406-994- 5150; http:// diagnostics.montana.edu/Plant/index.html for sample submission form and directions). 5 Exotic Exotic 6 Bull thistle Cirsium vulgare Canada thistle Cirsium arvense Biennial. Common to disturbed meadows, thickets and roadsides. Perennial. Common to moist, usually disturbed fields, meadows, thickets, roadsides, woodlands, or open forests.